A polyolefin-based microporous film has been widely used as a battery separator, a filter, and a membrane for microfiltration, due to its chemical stability and excellent physical properties.
Among methods to manufacture a microporous film from polyolefin, a wet process which makes a single phase melt by melt-mixing polyolefin with diluent at high temperature, phase-separates the polyolefin from the diluent during a cooling process and then forms pores by extracting the diluent is capable of preparing a thinner film and has excellent strength, permeability, uniform pores, and excellent quality stability, such that it has been widely used in a lithium ion secondary battery, etc.
Although the lithium ion secondary battery is an excellent battery having a very high energy density, it has danger of a fire or explosion when a short is generated, such that a separator used therein is greatly requested to have stability together with a high quality level. In accordance with the recent trend of high-capacity and high-power of the lithium ion secondary battery such as a battery for a hybrid vehicle, etc., there has been a greater demand for a thermal stability of the separator, in addition to the quality stability of the conventional wet product. This is the reason that if the thermal stability of the separator is deteriorated, the danger of explosion by the melt fracture of the separator according to overheat of the battery is increased.
The thermal stability of the battery is determined depending on a high-temperature shrinkage, a shutdown temperature and a melt fracture temperature of the separator.
If the separator is excessively shrinked at a high temperature, a surface of electrodes are exposed. In this case, the possibility of the occurrence of such an electrical short is increased, and thus the thermal stability of the battery is seriously deteriorated.
The shutdown temperature is a temperature that micropores of the separator are shutdown to shut-off the current, when the inside temperature of the battery is abnormally increased due to the electrical short and the like. The melt fracture temperature is a temperature that the separator is melt-fractured to allow current to be flowed again, when the temperature of the battery is continuously increased over the shutdown temperature. For the stability of the battery, it is preferable that the shutdown temperature is low and the melt fracture temperature is high. In particular, the melt fracture temperature, which is a temperature that can continuously block current in a situation that the explosion of the battery may be caused, has the closest relation with the stability of the battery.
The efforts to improve the thermal stability of the separator have been made continuously.
U.S. Pat. No. 6,949,315 discloses a film that improves thermal stability of a separator by melt-mixing inorganic material such as 5-15 wt % titanium oxide to ultra-high molecular weight polyethylene. However, this method may easily cause problems that melt-mixing ability is deteriorated according to the adding of the inorganic material, and pin-holes are generated and quality becomes non-uniform according to the deterioration of the melt-mixing ability when being stretched, and may cause deterioration in the physical property of the film such as impact strength, etc., due to a lack of compatibility in the interface between the inorganic material and polymer resin, despite the improvement in the thermal stability in accordance with the addition of the inorganic material. The separator using inorganic material cannot but have the disadvantages as described above.
A separator that is prepared by melt-mixing resin having excellent heat resistance instead of inorganic material is disclosed in U.S. Pat. No. 5,641,565. This technique is to mix 30-75 wt % organic liquid-phase compound and 10-50 wt % inorganic material with a resin mixture where polyethylene is mixed with 5-45 wt % polypropylene and then to extract the organic liquid-phase compound and the inorganic material, thereby preparing a separation membrane. Although this technique extracts the inorganic material, it still has the problems at the time of melt-mixing as described above and further causes a problem that physical property is deteriorated due to the addition of polypropylene that does not have melt-mixing ability with polyethylene as mentioned in the patent itself Also, this method is added with processes of extracting and removing the used inorganic material, thereby having a disadvantage that the process becomes complicated, and this method needs a relatively high amount of polyethylene in order to obtain a sufficient heat resistance, wherein the physical property of the separator is more deteriorated.
As a method to prepare a multilayer separator for improving the thermal stability of a separator, U.S. Pat. No. 5,691,077 discloses a method of making a 3 layer separator by laminating polypropylene having a high melt fracture temperature (having a high melting temperature) on polyethylene having excellent shutdown property (having a low melting temperature). Although this separator is excellent in view of thermal property, it has disadvantages of the non-uniform stretching, the generation of pin-holes, and the increase of thickness deviation, etc. during the preparation of a fabric film by a low temperature dry process, and also it has not only a deterioration problem in productivity due to the addition of the lamination process performed in a separate process but also a delamination problem due to the defect of the lamination, such that this separator has not been widely used. This method has problems that strength, permeability, quality uniformity, and productivity, which are indispensable for a separator of a secondary battery, are deteriorated, despite excellent heat resistance.
Japanese Patent Laid-Open Publication No. 2002-321323 discloses a microporous polyolefin multilayer film that a polyethylene film and a polyethylene/polypropylene compound film are laminated. However, since a polypropylene content of the polyethylene/polypropylene compound film is low, it is difficult to sufficiently increase the melt fracture temperature.
Japanese Patent Laid-Open No. 2007-088815 and PCT Publication No. WO2004-089627 disclose multilayer separators that have a microporous polyethylene film prepared using a wet process as a main layer and have a layer mixed with polyethylene and polypropylene prepared using a wet process as a surface layer. However, owing to its low crystallinity, when the polypropylene is used as a surface layer in the wetting process extruded with diluent, a lot of polypropylene wax remains in the diluent after being extruded and thus there is a problem that a film and a roll surface are polluted by the wax extracted during post-processings such as stretching/extrusion, etc, thereby deteriorating quality stability.
Furthermore, in these technologies, a shrinkage of film, which is one of the most important factors in the thermal stability, is left out of consideration.
PCT Publication No. 2006-038532 discloses a multilayer separator by wet process containing inorganic material. However, this separator also has a complicated melt-mixing process due to the melt-mixing of inorganic material. Also, if inorganic material is added into the surface layer, the inorganic material is separated during processes of stretching/extraction/winding/slitting, etc. so that it may cause pollution due to inorganic powder and scratch of other surface layers, thereby deteriorating quality stability.
The indispensable properties of a separator for a secondary battery are strength, permeability and quality uniformity, and in recent, there is a great demand for thermal stability additionally. However, the conventional techniques as described above have not accomplished the quality stability, the strength/permeability and a high thermal stability at the same time.